Fish assemblage composition in constructed and ... - Springer Link

Estuaries Vol. 22, No. 3A, p. 702-716 September 1999

Fish Assemblage Composition in Constructed and Natural Tidal Marshes of San Diego Bay: Relative Influence of Channel Morphology and Restoration History GREGORY D. WILI.IAMS 1 J o Y B. ZEDLER 2

Pacific Estua~ine Research Laboratory San Diego State University San Diego, California 92182-1870 ABSTRACT: This study evaluated the use by fish of restored tidal wetlands and identified links between fish species composition and habitat characteristics. We compared the attributes of natural and constructed channel habitats in Sweetwater Marsh National W'ddlife Refuge, San Diego Bay, California, by using fish monitoring data to explore the relationships between channel environmental characteristics and fish species composition. F'mhes were ~ - n p l e d annually for 8 yr (1989-1996) at eight sampling sites, four in constructed marshes and four in natural marshes, using beach seines and blocking nets. We also measured channel habitat characteristics, including channel hydrology (stream order), width and maximum depth, bank slope, water quality (DO, temperature, salinity), and sediment composition. Fish colonization was rapid in constructed channels, and there was no obvious relationship between channel age and species richness or density. Total richness and total density did n o t differ significantly between constructed and natural channels, although California killifish (Fundu/us parv/p/nn/s) were found in significantly higher densities in constructed channels. Multivariate analyses showed fish assemblage composition was related to channel habitat characteristics, suggesting a channel's physical properties were more important in determining fish use than its restoration status. This relationship highlights the importance of designing restoration projects with natural hydrologic features and choosing proper assessment criteria in order to avoid misleading interpretations of constructed channel success. We recommend that future projects be designed to mimic natural marsh hydrogeomorphology and diversity more closely, the assessment process utilize better estimates of fish habitat function (e.g., individual and community-based species trends, residence time, feeding, growth) and reference site choice, and experimental research be further incorporated into the restoration process.

Introduction

species (52% by weight in the Pacific Northwest; 18% in California) (Pearcy and Myers 1974; Emmett et al. 1991; Chambers 1992). In smaller lagoons of southern California the occasional closure of ocean inlets exacerbates harsh environmental conditions and limits their function as migration corridors and nursery habitats for marine species (Carpelan 1961; Nordby and Zedler 1991). Still, coastal marsh fishes in southern California are valued for their contribution to regional biodiversity (Onuf et al. 1979; Swift et al. 1993) and food chain support functions (Horn and Allen 1985; Kwak and Zedler 1997). These coastal wetlands provide critical habitat for isolated and genetically unique fish populations (e.g., federally endangered tidewater goby, Eucyclogobius newberryi; Swift et al. 1989) and for resident and migratory birds, including several species (e.g., federally endangered California least tern, Sterna albifrons browni) that feed primarily on shallow-water fishes (Zedler 1996a). Wetland restoration and creation techniques are increasingly being used to ameliorate wedand habitat losses, especially in regions where h u m a n im-

Tidal wedands function as important habitats for fishes and other biota (Weinstein 1979; Boesch and Turner 1984). In the United States, the extent of these wetlands and their associated functions and relative values vary by geographic region (Field et al. 1991; Zedler 1996a). It is estimated that 94-98% of the commercial fishery catch by weight for the expansive wetlands of the Southeast and Gulf of Mexico is made up of estuarine-dependent species that depend on these habitats for reproduction, nursery areas, food production, or migration (Chambers 1992). Pacific coastal wetlands are generally small in area (3% of the United States' total; Field et al. 1991), experience environmental extremes due to high seasonal and interannual variation in precipitation and runoff (Zedler 1996a), and have a smaller percentage of commercially important, estuarine-dependent, fishery ~Corresponding author; tele: 619/594-7436; fax: 619/5942035; e-mail: [email protected]. 2 Current address: University of Wisconsin Madison Arboreturn, 12{)7 Seminole Highway, Madison, Wi~onsin 53711. 9 1999 Estuarine Research Federation

702

Fish Assemblage Composition in Marsh Channels

pacts have been extensive (see Kusler and Kentula 1990; Zedler 1996b). In southern California, which has the nation's highest loss of historic coastal wetland area (90%) (National Oceanic and Atmospheric Administration 1990), harbors with breakwaters have largely replaced estuaries as inshore fish habitats (Horn and Allen 1985). Although numerous marsh restoration and creation projects have been implemented, experience shows they have been unsuccessful in providing self-sustaining ecosystems that closely resemble natural systems in both structure and function (Zedler 1996a). Assessments of habitat functional equivalency are characteristically based on short-term measurements of wetland ecosystem structure rather than on long-term studies of processes (Simenstad and Thorn 1996; Zedler 1996b). For example, while functional support of fish populations is often a major goal of individual projects, most monitoring plans rely upon composite summary measures of total species diversity and abundance to .judge successful fish habitat use (City of Carlsbad and United States Army Corps of Engineers 1990; Simenstad et al. 1991; Minello and Zimmerman 1992; MEC Analytical Systems, Inc. 1993, 1995; Havens et al. 1995; Zedler et al. 1997). Critical evaluation of coastal habitat restoration efforts requires long-term, comparative research on the structure and function of sites with different alteration histories (e.g., disturbed, natural, created, and restored) over a range of physical conditions (Thayer et al. 1996). A recent project in San Diego Bay, California, fulfills most of these criteria. Starting in 1984, channel habitats in Sweetwater Marsh National Wildlife Refuge (Fig. 1) were designed and excavated by the California Department of Transportation (Caltrans) to mitigate the impacts of widening a freeway (I-5), constructing a highway interchange (SR-54), and excavating the Sweetwater River flood control channel. The United States Fish and Wildlife Service required that the created marshes provide foraging area for the endangered California least tern. As a result, restoration criteria were established for marsh fish assemblages. These criteria stipulated that constructed channels provide 75% of the species richness and 75% of the total density of selected natural (reference) channels for two consecutive years (United States Fish and Wildlife Service 1988). Although the project had complied with these requirements by the second year, monitoring continued to be funded partly because of concerns related to the abundance of nonindigenous fishes. Fishes were sampled annually for 8 yr (1989-1996) at eight sampling sites, four in constructed marshes and four in natural marshes. Long-term data allow us to extend our evaluation

703

]\ ...... !., .J.,

oe

i East West

s~ Street

Fig. 1. Sweetwater Marsh National Wildlife Refuge (32~ 117~06'W) fish sampling stations (C = constructed, N = natural). Shaded areas represent existing marsh habitat. Map compiled from August 1994 ADAR (airborne data acquisition and registration) flight,

of habitat equivalency beyond the assessment of compliance with mitigation criteria. Wetlands constructed by excavating tidal channels are likely to have very different shapes and ratios of creek sizes than those evolving over centuries. Few studies have examined the relationship between wetland channel morphology and fish functions. A predictive knowledge of the relationship between wedand channel morphology and fish use is needed in order to design and construct wetland habitats that sustain natural fish assemblages. Because aquatic habitat structure and complexity affect species composition and function (e.g., Sebens 1991; Hixon and Beets 1993), fish assemblages may be good indicators of habitat development. Fish assemblage composition has been used as a correlate for habitat quality and for determining successful habitat restoration in stream and reef ecosystems (Karr 1987; Fausch et al. 1990; Bortone and Kimmel 1991).

704

G.D. Williams and J. B. Zedler

We used 8 yr of environmental and fish assemblage data to compare natural and constructed channel habitats in the Sweetwater Marsh wetland complex, and to predict functional processes. First, habitat features were contrasted across sampling sites. Second, fish colonization rates and assemblage composition (total abundance and species richness), two summary measures commonly used to assess habitat function, were examined. Third, individual- and multi-species abundance patterns, which may better reflect habitat function, were compared. Finally, we integrated environmental and biological variables by investigating correladons between fish assemblage composition and channel habitat characteristics. With a new understanding of relationships between marsh channel structure and biotic assemblages, we suggest guidelines for more biologically meaningful coastal saltmarsh restoration plans and designs. Methods

SITE DESCRIPTION Sweetwater Marsh National Wildlife Refuge (32~ 117~ encompasses a 128-ha patchwork of salt marsh channels and vegetation, intersected by a number of bermed roadways, dikes, dredged flood control channels, and other anthropogenic alterations (Fig. 1). It makes up the majority of the --15% of historic natural wetland habitat area remaining in San Diego Bay (Mudie 1970; MacDonald 1990). Natural habitat is a relative term along the southern California coast--all marsh areas of San Diego Bay are affected by urban d e v e l o p m e n t , i n c l u d i n g a u g m e n t e d flows from street drains, decreased peak flood flows from an upstream dam, reduced water quality, and altered morphology due to historical dredge spoil deposition (Haltiner et al. 1997). Selection of our monitoring sites within the Refuge was not random but was constrained by the lack of adequate, accessible areas where regular trampling of the marsh could be minimized. We selected the few good places where it was feasible to repeatedly sample fishes and habitat parameters without destroying tiny remnants of marsh. Four natural reference channels N1-N4 were established in 1989. Isla Flaca (N1) is located within the historic channel of the Sweetwater River; Sweetwater Marsh (N2) is on a small, side channel of the Sweetwater River surrounded by a large contiguous saltmarsh; E St. Marsh (N3) is a small tidal creek leading directly into the bay; and F & G St. Marsh (N4) is bounded by dredge fill and has tidal circulation reduced by a culvert under a city street (Fig. 1). Four permanent sampling sites were established in channels that had been constructed as

part of the Caltrans mitigation setdement. Three of these (C1-C3) were established in 1989 at the 12-ha C o n n e c t o r Marsh, a series of c h a n n e l s dredged in 1984 to form several marsh islands (Fig. 1). The fourth constructed sampling site was established in 1991 at Marisma de Naci6n (C4), a meandering tidal channel with 8 ha of associated saltmarsh that was excavated from dredge fill and opened to tidal flow in February 1990. CHANNEl, HABITAT CHARACTERISTICS

Physicochemical measurements of channel characteristics were made at the same time as fish sampling. Channel width and maximum water depth were measured with a meter tape. Dissolved oxygen and temperature measurements were made at the bottom and surface of the water column using a Yellow Springs Instruments (YSI) model 51B DO/temperature meter. Water salinity was measured to the nearest part per thousand using a YSI model 33 Salinity-Conductivity-Temperature meter or a temperature-compensated refractometer. Instruments were regularly calibrated and checked for accuracy. Sediment composition of each site was examined in February 1997 by taking three replicate 10cm diam cores of the top 5 cm of sediment from the bottom of each channel and analyzing these to determine grain size and texture (Gee and Bauder 1986). Sediment composition was based on three grain-size classes: sand (> 50 ~zm particle diam), silt (< 50 p~m and > 2 ~m), and clay (< 2 ~m). Proportional representation of component size classes was calculated for each sample. Channel profiles of all sites were surveyed in February 1997 by taking elevation measurements (using a surveyor-grade autolevel, tripod, and stadia rod) at 0.5-1.0 m intervals in transects running perpendicular to channels. Bank slopes were calculated (Coats et al. 1995) by measuring the distance between the lower edge of bank vegetation and maximum channel depth (horizontal distance) and the elevation difference between these two points (vertical distance). Natural channel sites were categorized by stream order (Strahler 1964) using aerial photographs. A two-way fixed factor ANOVA without interactions was used to test whether channel status (natural versus constructed) and sampling year (19891996) affected physicochemical parameters (depth, width, temperature, salinity, and dissolved oxygen [DO]) measured during fish sampling. Data were log(x + 1) transformed when necessary to meet assumptions of normality and homogeneity of variance (Underwood 1981; Zar 1984). Ttests were used to evaluate differences in 1997 sediment composition and mean bank slope across

705


TABLE 1. Fish s u m m e r s a m p l i n g dates (June-July) at na t ura l a n d c r e a t e d tidal c h a n n e l s of Sweetwater Marsh, 1989-1996 (n = 51); * s a m p l e t a k e n b u t o m i t t e d from analysis due to i n c o n s i s t e n t m e t h o d s . Natural Sites

1989

1990

1991

1992

1993

1.094

1995

1996

C1----Connector Marsh N o r t h C 9 - - - C o n n e c t o r Marsh West C~---Connector Marsh East C4---Marisma de Nacion

X X X

X X X

X X X X

* * X X

X X X X

X X X X

X X X X

X X X X

X X X X

X X X X

X

*

X

X

X

X

X

X

X X X X

X X X X

Constructed

Natu ral N 1 - - I s l a Flaca N2---Swee twater Marsh N.%---E Street Marsh N4---F & G Street Marsh

natural (n = 4) and constructed sites (n = 4). Graphical techniques were used to g r o u p sites with similar habitat characteristics. FISH ASSEMBLAGES

Adult and juvenile fishes were collected during slack periods o f low n e a p tides at each o f the sampiing sites using two blocking nets and o n e 15 m long by 2 m d e e p bag seine, all c o m p o s e d o f 3-mm square delta mesh. At each study site, a linear distance o f approximately 10 m was m e a s u r e d parallel to the c h a n n e l and blocking nets were deployed to confine all fishes within this area. T h e bag seine was swept between the two blocking nets and across the c h a n n e l to the opposite bank (one pass). Passes were r e p e a t e d until the n u m b e r o f fish c a p t u r e d p e r pass a p p r o a c h e d zero. T h e species composition and n u m b e r o f fishes, c o m b i n e d across all passes, were r e c o r d e d for that site (one sample). Fish collections at all sites were generally m a d e within a 1-wk p e r i o d because o f the time limitations i m p o s e d by the short period of low, slack tide (generally 1-2 h). T h e two created c h a n n e l systems were excavated at different times, and fish m o n i t o r i n g began at two intervals thereafter. At the C o n n e c t o r Marsh channels (C1-C3) m o n i t o r i n g began in 1989, 5 yr post-construction, while at Marisma de Naci6n (C4) m o n i t o r i n g c o m m e n c e d in 1991, 1 yr postconstruction. Fish sampling in natural channels (N1-N4) was b e g u n in 1989 to serve as a r e f e r e n c e for the C o n n e c t o r Marsh mitigation project. Monitoring c o n t i n u e d quarterly (seasonally) at C1-C3 and N 1 - N 4 for two full years until 1991, when fish mitigation criteria m e t compliance r e q u i r e m e n t s (constructed channels m e t 75% o f the total native fish species richness and density estimates f o u n d in natural r e f e r e n c e channels for two consecutive years; U n i t e d States Fish and Wildlife Service 1988). Thereafter, sampling effort and frequency was r e d u c e d to a single s u m m e r sample at all sites

(including C4), primarily to m o n i t o r exotic species (Table 1). S u m m e r samples (1989--1996) at all sites were m a d e over a 1-wk p e r i o d between late J u n e and early July. For the purposes o f this study, analysis o f species diversity and density data was c o n f i n e d to 51 summ e r samples m a d e at eight sites over 8 yr (Table 1). Sampling at two sites (N2 and N4) was completely discontinued f r o m 1991 to 1994, although all eight original sites were resampled in the summers o f 1995 and 1996. T h r e e samples collected in s u m m e r 1992 (C1, C2, N1) were d r o p p e d f r o m the dataset because of inconsistent methods. To measure fish habitat functions across natural and constructed sites, we c o m p a r e d several parameters: colonization rates, total a b u n d a n c e and species richness, and individual species abundance. Colonization rates f r o m constructed sites were g r a p h e d using species richness and total density o f fish versus relative channel age (years post-construction). Species richness, which is strongly "affected by sample size and area (Magurran 1988; Fausch et al. 1990), was standardized by unit area; m e a n values from natural r e f e r e n c e sites were g r a p h e d for comparison. A two-way fixed factor ANOVA was used to test w h e t h e r c h a n n e l status (natural versus constructed) a n d / o r year o f collection (1989-1996) significantly affected total fish d e n s i t y a n d species r i c h n e s s ( s t a n d a r d i z e d by area). Although we were interested primarily in determining differences across natural and constructed sites, year o f collection was included as a factor in the ANOVA in o r d e r to partition out interannual variance, which is often substantial in fish populations. Using identical ANOVA methods, a b u n d a n c e patterns o f c o m m o n individual species (average densities > 0.01 m -2) w e r e c o m p a r e d . S P E C I E ~ H A B I T A T ASSOCIATIONS

We used multivariate m e t h o d s that m e a s u r e d and g r o u p e d samples based on the similarity o f bi-

706


otic variables (relative fish abundance) and then examined patterns in the abiotic variables associated with these groupings. Cluster analysis was used to evaluate broad patterns in species abundance over sites and years. A total of 51 fish samples from eight sites, collected over 1989-1996, was included in this classification. Each sample was relativized (by the proportion of each species in a sample) to reduce the influence of highly variable interannual density estimates and to highlight similarities in the relative composition of the assemblage. Bray-Curtis intersample similarities were calculated and agglomerative hierarchical clusters were formed using an average-linkage algorithm (Piepenburg and Piatkowski 1992; SYSTAT 1992). Mean environmental parameters (physicochemical data) were calculated from samples within each of the major groups differentiated by the cluster analysis. Environmental characteristics (depth, width, salinity, temperature, DO) were compared across groupings using a single factor ANOVA, with significant differences calculated by Tukey multiple comparison tests (Zar 1984). Canonical correspondence analysis (CCA; ter Braak 1988) was used to further confirm relationships between fish species assemblage composition and environmental characteristics (salinity, temperature, DO, and channel depth and width) measured with each sample. Environmental variable, species composition, and sample site (location and year) coordinates were graphed along ordination axes extracted by CCA and interpreted for relationships (ter Braak and Verdonschot 1995). The graphs show the explainable variation in faunal composition and site characteristics along horizontal and vertical axes, with species locations summarizing the niche centers of species along each environmental variable. Results C H A N N E L H A B I T A T CHARACTERISTICS

As a group, natural channels were significantly narrower (p < 0.04), had higher mean salinities (p < 0.002), and lower mean DO concentrations (p < 0.01) than constructed channels (Fig. 2a,b). No differences were detected in depth or temperature by channel status (p > 0.7); only temperature was significantly different by year (p < 0.01). Mean bank slope and sediment composition (p > 0.15) showed no significant differences by channel status (Figs. 2c and 3). Sampling sites were represented by a wide range of channel configurations (as measured by channel width, maximum depth, bank slope), water quality (DO, temperature, salinity), and sediment composition characteristics that varied both across

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Fig. 2. Habitat characteristics o f c h a n n e l stations C 1 - C 4 a n d N 1 - N 4 (C = constructed, N = natural); values are m e a n s (+- 1 SE): A. width a n d depth, 1989-1996 (n = 8); B. salinity (%0), t e m p e r a t u r e (~ a n d DO (rag 1-1) 1989-1996 (n = 8); C. perc e n t c o m p o s i t i o n o f b o t t o m s e d i m e n t , 1997 (n = 3).

and within c o n s t r u c t e d and natural channels. Three of the natural channels selected as reference sites were narrow (< 6 m) and shallow (< 0.8 m), and could be categorized hydrologically as secondorder channels (N2-N4; Figs. 1, 2a, and 3). The fourth natural reference site, Isla Flaca (N1), was a wide (> 12 m), fourth-order channel located in the main stem of the historic Sweetwater River channel. Bank slopes of the natural channels varied substantially as a function of a site's hydrologic classification, with the steepest slopes (2 : 1 to 10 : 1 H:V) in most low-order channels, and gradual slopes (> 14:1 H:V) in the single, high-order channel (Fig. 3). Surface sediments at the bottom of most natural channels were composed of a high proportion of fine clay and low proportion of coarse, sandy particles compared to constructed channels (Fig. 2c). Constructed wetlands lacked small tidal creeks, so stream order was not useful in their description. Three of the constructed channel sites (C1-C3) chosen for monitoring were relatively wide (> 6 m), high-flow channels with coarse sediments and gradually sloping banks (none steeper than 4 : 1 H : V; Figs. 2a,c and 3). The fourth site (C4), located within the terminal tidal system of Marisma de Na-

Fish Assemblage Composition in Marsh Channels CONSTRUCTED

NATURAL 5

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15

20

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25 30 35 NI - Isla Flaca

10 CI

15

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- Connector

25 30 35 Marsh Not fit

0.5: 0.0

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707

us) and sailfin molly (Poecilia latipinna). N e i t h e r species was particularly a b u n d a n t , making up less than 2% (1.4% yellowfin goby, 0.1% sailfin molly) o f the total catch, although b o t h have b e e n prevalent in Sweetwater Marsh collections since the study was initiated in 1989, with yellowfin gobies o c c u r r i n g in over half o f all samples (Table 2). COLONIZATION OF CONSTRUCTED CHANNELS

0.5 Z~ 0.0 . . . . . . . . . .

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0.01.-- -~.24m . . . . . . . . . . . . .

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C4 - M'.r~m'. de l~.cion

~. . . . . . . . . . .

Distance(m) Fig. 3. Channel cross sections (natural reference channels on left, constructed channels on right) at Sweetwater Marsh National Wedand Reserve, 1997. Open points represent areas with emergent vegetation; horizontal dashed lines represent 0.0 m National Geodetic Vertical Datum (NGVD), channel bottom elevations are denoted in bold. Bank slope values (ratio of horizontal to vertical distance) are adjacent to graphed slope.

ci6n, was a sinuous c h a n n e l with relatively steep banks a n d a high p e r c e n t a g e o f clay sediments. In terms o f the quantified habitat characteristics, C4 differed the most f r o m the constructed Conn e c t o r channels (C1-C3) and m o r e closely resembled the low-order, natural sites (N2-N4). In comparison, the single, high-order, natural, r e f e r e n c e c h a n n e l N1 was m o r e similar to the high-flow constructed channels (C1-C3). FISH ASSEMBLAGES

A total o f 13,220 individual fishes, representing 17 species, was collected f r o m all marsh c h a n n e l sites over the course o f eight s u m m e r sampling periods (1989-1996). Mean total channel density equaled 3.33 - 0.5 individuals m -2 (Table 2; all measures of variability d e n o t e d as standard errors [SE] in text). F o u r native species, topsmelt (Atherinops affinis), California killifish (Fundulus parvipinnis), lonKjaw m u d s u c k e r ( Gillichthys mirabilis), and arrow goby (Clevelandia its), c o m p o s e d over 93% o f all collections and o c c u r r e d in most seine samples. Catches also included two n o n i n d i g e n o u s species, the yellowfin goby (Acanthogobiusflaviman-

Fish colonization was rapid at Marisma de Naci6n (C4), which was the only site m o n i t o r e d in the first 4 yr following construction (Fig. 4). Nine species (0.18 species m -2) were collected at densities o f 23.4 individuals m -2 in the first year postconstruction (Table 2 and Fig. 4). These fish densities far e x c e e d e d estimates m a d e at any o t h e r site during the 8-yr m o n i t o r i n g period; n o o t h e r sample e x c e e d e d 10 individuals m -2, while average densities at natural sites across all years was 2.3 individuals m z. Species richness and density estimates were highly variable at C4 and generally declined in subsequent years (2-6 yr post-construction), never again reaching first year levels. At the constructed sites that were m o n i t o r e d f r o m 5 yr to 12 yr post-construction (C1-C3), fish species richness and density were highly variable and displayed little relationship with c h a n n e l age. CONSTRUCq'ED VERSUS NATURAL REFERENCE CHANNELS

By 1991, constructed channels for two consecutive years had m e t or e x c e e d e d 75% o f the native fish species richness and density f o u n d in reference channels, and the project was j u d g e d in compliance with mitigation criteria at the earliest date possible (Fig. 5, Haltiner et al. 1997). Seventeen species were collected f r o m constructed channels over all years (1989-1996), while 14 species were collected f r o m natural channels (Table 2). T h e three species n o t f o u n d in natural channels were striped mullet (Mug~l cephalus), n o r t h e r n anchovy (Engraulis mordax), and r o u n d stingray (Ur0lophus hal/en). Constructed channels (n = 28) were sampled m o r e than natural channels (n -- 23), and seine collections m a d e in constructed channels also sampled a substantially h i g h e r average area (constructed: 93 m 2 versus natural: 75 m2). W h e n standardized by area, m e a n species richness o f constructed channels (0.078 -+ 0.008 species m -z) was not significantly different f r o m natural channels (0.086 +- 0.01 species m -z) in any year (p > 0.2). Total fish density r a n g e d from 0.3 individuals m -2 to 23.4 individuals m-2 per sample over the 8yr m o n i t o r i n g period. Preliminary examination o f the data showed h e t e r o g e n e o u s variances across natural and constructed sample groups (Bartlett test; X2 = 25.2, p < 0.001) as well as a single outlier,

708


TABLE 2. Fish species composition and m e a n density (m 2) from s u m m e r samples (June, July) taken in natural and created tidal channels of Sweetwater Marsh, 1989-1996. Species ranked by total mean density; * denotes exotic species. Natural Marsh Species C'~.lifornia killifish Topsmelt Long~aw m u d s u c k e r Arrow goby Bay pipefish Yellowfin goby Cheekspot goby Shadow goby Bay goby Striped mullet D i a m o n d turbot Sailfin molly California halibut N o r t h e r n anchovy Deepbody anchovy R o u n d stingray Staghorn sculpin

Fundulus parvipinnis Atheffnops affinis Gillichthysmirabilis Clevelandia ios Syngnathus leptorhynchus Acanthogobiusflavimanus* llypnus gilberti Quietula y-cauda Lepidogobius lepidus Mugil cephalus Hypsopsetta guttulata Poecilia latipinna* Paralichthyscalifornicus Engraulis mordax Anchoa compressa Urolophus hallen I~Otocottusarmatus Mean total density (m2): Total species richness:

1989 (n = 4)

1990 (n = 4)

1991 (n = 2)

1992 (n = 1)

1993 (n = 2)

1994 (n = 2)

0.42 0.90 1.80 0.93 0.03

0.01 1.25 1.03 0.11 < 0.01 0.01

0.18 1.34 0.87 0.50 0.02 0.08 0.09

0.33

0.39

0.60

0.40 0.35 0.17 0.05 0.02

0.31 0.85 0.80 0.50 0.03 0.20

0.13

0.09 0.00 < 0.01

0.02

1996 (n = 4)

0.07 0.18 0.92 0.06 0.04 0.02 0.04

0.47 0.21 0.48 0.25 0.03 0.02 < 0.01 0.05

< 0.01 < 0.01

< 0.01 0.01 < 0.01

1.34 9

1.53 11

0.01

0.00

< 0.01

1995 (n = 4)

0.02 0.01

< 0.01 < 0.01 4.30 12

C4, in 1989 (see Colonization of Constructed Channels section), that was one order of magnitude removed from the data. Data were log(x + 1) transformed and the outlier d r o p p e d to meet ANOVA assumptions. No significant differences were detected in m e a n density by channel status (backtransformed; constructed channels -- 4.15 - 0.85 individuals m-2; natural channels = 2.33 - 0.29 individuals m -2) in any year (p > 0.14). While the extremely high densities of the outlier site may hold biological significance beyond this comparison, our results are supported by preliminary ANOVA tests r u n before this data point was excluded (p > 0.1). On an individual species basis, killifish had the highest m e a n density (1.4 _+ 0.3 individuals m --2) of any species in constructed channels, while mudsuckers were the most a b u n d a n t species (0.9 -+ 0.2 individuals m 2) in natural channels (Fig. 6). Killifish were the only c o m m o n species (see Table 2) exhibiting a b u n d a n c e differences by channel status, with significantly higher densities in constructed channels than in natural channels (p < 0.001). No species was more a b u n d a n t in natural than constructed channels, a n d we did n o t detect differences a m o n g sampling year for any species (p > 0.09). There was no evidence that yellowfin gobies, a c o m m o n n o n i n d i g e n o u s species, m a d e greater use of constructed than natural channels (p > 0.45; Table 2); sailfin mollies were not a b u n d a n t e n o u g h in samples to test statistically. 8 P E C I E ~ H A B I T A T ASSOCIATIONS

Cluster analysis of the 51 samples, pooled over all sites and years, yielded four groupings, each

2.42 7

3.10 9

0.93 2

1.41 8

2.68 6

characterized by one numerically d o m i n a n t species (Fig. 7 and Table 3). The first group, hereafter called the mudsucker assemblage, included 18 samples that were d o m i n a t e d by long~aw mudsuckers (mean percentage = 66 -+ 3%). The second group, the killifish assemblage, included 16 samples d o m i n a t e d by killifish (63 -+ 5%), and the third, topsmelt assemblage, included 15 samples d o m i n a t e d by topsmelt (55 -+ 6%). The final group had only two samples, both collected in 1989, which were d o m i n a t e d by arrow gobies (68 + 4%). Because data were inadequate to characterize this group, we sought habitat relationships for only the first three groups. Fish assemblage composition varied little within a site on an interannual basis, with classification in consecutive years remaining the same 53% of the time (20 of 38 possible consecutive sampling periods). Composition of the fish assemblage corresponded to channel habitat characteristics. Longjaw mudsuckers were generally the d o m i n a n t species in shallow channels (0.6 + 0.08 m) with significantly narrower banks (6.3 -+ 0.8 m; p = 0.013; Tukey multiple comparison test, p < 0.05) and higher salinities (38.1 -+ 1.2%o; p = 0.007; Tukey's, p < 0.03) than habitats of the other two assemblages (Fig. 8). Samples in the mudsucker assemblage were represented almost exclusively by loworder, natural reference sites, including samples from N3 for eight consecutive years and from N4 for three of four years (Fig. 7). California killifish were generally the most prevalent species in relatively broad (9.4 -+ 0.8 m), shallow (0.5 -+ 0.1 m) channels (Fig. 8). The killifish assemblage was rep-

Fish Assemblage Composition in Marsh Channels T A B L E 2.

Extended. ConstructedMarsh

1989 (n = 3)

1990 (n = 3)

0.29 2.22 0.50 0.22 0.16 0.08

1991 (n = 4)

0.88 1.30 0.56 1.55 0.11 0.15

1992 (n = 2)

0.24 3.00 0.73 2.78 0.10 0.14 0.55 0.16 0.27

< 0.01 < 0.01 0.04 < 0.01

1993 (n = 4)

0.60 0.56 0.11

3.71 0.56 0.65 0.05 0.06 0.03

0.03 0.01

Total 1994 (n = 4)

1995 (n = 4)

1996 (n = 4)

2.57 0.22 0.32 0.44 < 0.01 0.02

1.40 0.28 1.76 0.11 0.06 0.10

0.99 0.12 0.42 0.15 0.09 < 0.01 < 0.01 0.07

0.07 < 0.01

0.01

< 0.01

< 0.01 < 0.01

0.01

< 0.01 0.01

< 0.01

< 0.01 0.01

< 0.01 0.O9 < 0.01

< 0.01 < 0.01

3.52 10

4.55 6

7.99 12

1.39 6

5.09 10

resented almost exclusively by constructed sites, and included samples from C3, one of the shallowest, most gradually sloped channels (Figs. 2 and 3), for six of the 8 yr sampled (Fig. 7). Habitats of the

~"

E

0.2 A 0.15

A

Natural Site Mean +_S.E.

A [] ~

o

o

0.1

~.0 v

0.0s

A

~x~

r

0 0

[]

25-

o [] o ~"

10

o D

~

709

e

o

c1 c2 C3 C4

Meanfor Constructed channels

5o

i

i

i

i

i

i

i

o

i

i

i

i

Age of Channel(Years)

Fig. 4. ()olonization rates of constructed channel sites (C1(]4) at Sweetwater Marsh c o m p a r e d against natural site m e a n species richness (rn -2) and density ( m - 2 ; + 1 SE indicated by shaded area). C1-C3 are located in C o n n e c t o r Marsh, constructed in 1 9 8 4 ; (]4 is in Marisma de Naci6n, consu'ucted in 1 9 9 0 . M e a n for natural c h a n n e l s is based o n 23 samples collected f r o m a m o n g f o u r sites over 8 yr ( 1 9 8 9 - 1 9 9 6 ) .

3.59 10

3.79 8

Mean Density (n = 51)

< < < < < < <
1% o f the total catch; bold taxa represent numerically d o m i n a n t species assemblages.

signing restoration projects with natural hydrologic features and choosing proper assessment criteria in order to avoid misleading interpretations of constructed channel success. Habitat associations (e.g., Bahz et al. 1993; Paller 1994) of the dominant fish species (longjaw mudsucker, California killifish, arrow goby, and topsmelt) were identified and these suggest tidal channel habitat characteristics influence fish assemblage composition. Fish catches dominated by longjaw mudsuckers came from narrow, low-order channels characterized by high salinities, low DO levels, and steep, clay banks. These large gobioids typically are associated with the bottom in shallow, muddy backwaters with a high density of crab burrows and dense macroalgae ( Ulva sp., Enteromorpha sp.). Mudsuckers have the ability to respire by gulping air into their buccal cavities when water quality declines (Todd and Ebeling 1966), and, consequently, can tolerate occasional periods of low water quality and emersion in poorly flushed or intertidal habitats. In comparison, fish assemblages

712


at channels with broad, gradually sloping banks, sandy sediments, and shallow waters were dominated by California killifish, and to a lesser degree, arrow gobies. Arrow gobies are abundant on intertidal mudflats, where they live commensally in the burrows of several b e n t h i c invertebrate hosts (Brothers 1975). California killifish are usually associated with shallow, vegetated, marsh habitats (Fritz 1975; Desmond et al. In press). They are one of the dominant species found at high tide on the vegetated marsh surface, where they feed on a variety of terrestrial arthropods (Johnson 1999) and are thought to spawn (Fritz 1975). Fish catches at deeper channels with higher tidal flows were dominated by topsmelt, a pelagic schooling species found in nearshore habitats (Miller and Lea 1972). These resident estuarine fishes were the numerically dominant species in surveys of several other southern California coastal wetlands (Allen 1982; O n u f and Quammen 1983; Horn and Allen 1985; Nordby and Zedler 1991; Yoklavich et al. 1991). Fish rapidly colonized new habitats in high numbers, but our assessments did not provide evidence that the fish assemblage developed in a linear fashion through time (1-12 yr after tidal channels were constructed). Observations from other studies confirm that (re)colonization of hydrologically restored or created wetland habitats is rapid, and indicate that fishes exploit new habitats more quickly than some other less motile organisms (e.g., benthic invertebrates) (Moy and Levin 1991; Minello and Zimmerman 1992; Vose and Bell 1994; Simenstad and T h o m 1996). Simenstad a n d Thorn (1996) observed fish species richness and density at a created wetland approached asymptotes within 3-5 yr, while infauna taxa richness and density remained depressed over the same period. Minello and Zimmerman (1992) documented no significant differences in overall fish density between natural and 2-5 year-old restored salt marshes in Texas, while total invertebrate diversity and density were lower in the transplanted marshes. Our observations affirm Zedler and Callaway's (1999) contention that many mitigation sites may not follow trajectories (Kentula et al. 1992; Simenstad and Thorn 1996). Site hydrology appeared to be the driving force in determining a channel's physical characteristics, regardless of its restoration status (natural versus constructed). However, study site selection was constrained due to the availability of adequate reinnants of accessible natural marsh, and site hydrology was neither controlled for in the study design nor was it weighted in the original assessment criteria. Hydrologic conditions were disproportionately represented across both natural reference and constructed channels, leading to a broad

range of habitat features (channel morphology, hydrology, chemistry) and aspects of habitat heterogeneity. Natural reference sites were generally represented by low-order, narrow, steeply banked channels with muted tidal flushing. In contrast, constructed channels were primarily sited in highflow areas and had broad, gradually sloping banks that provided habitat important to a subset of the resident fish community. Based on our analysis of long-term data, we suggest guidelines for marsh restoration design and assessment. Marsh habitat restoration plans need to be designed with goals of replicating natural structure and function. This design process should incorporate an understanding of the relationship between fish species habitat associations, hydrology, marsh channel morphology, and knowledge of natural habitat ratios and diversity. Assessment procedures should carefully consider the selection of reference sites, criteria used to quantify fish function, and monitoring time period, and attempt to integrate basic experimental research that will improve future projects. DESIGN Habitat is a species-specific concept, and a quantitative knowledge of species-habitat relationships will assist in the development of models that predict the effects of habitat creation a n d / o r restoration designs on fish composition and function (Fausch et al. 1990; Harris 1995). Likewise, this knowledge will improve goals for future projects. Several marsh studies underscore the importance that habitat features have in determining fish assemblage composition and function. Mclvor and Odum (1988) demonstrated that tidal marshes adjacent to gradually sloping, depositional creek banks had higher fish densities than those adjacent to steep, erosional banks. They suggested subtidal geomorphology in marsh channels might influence observed intertidal fish distribution patterns, possibly by mediating prey availability and predator encounter rates (Mclvor and Odum 1988). In Gulf of Mexico salt marshes, the distribution and abundance of small fishes was correlated to the proximity and type of channel vegetation, water depth, and salinity (Baltz et al. 1993; Peterson and Turner 1994). Species-specific habitat associations in other aquatic settings have been related to such factors as habitat structural composition, flow velocity, depth, stream order, wave exposure, and turbidity (Gorman and Karr 1978; Allen 1985; Meffe and Sheldon 1988; Ruiz et al. 1993; Paller 1994; Kirchhofer 1995; Clark et al. 1996). Marsh projects should be designed to mimic natural marsh geomorphology as closely as possible, with care taken to heed hydrologic conditions that


provide the foundation for marsh development and maturation (Zedler 1996a,b). Marsh sediments, plant roots, and hydraulic effects of tidal flow are the primary physical factors that determine a tidal marsh channel's morphology (Garofalo 1980; Frey and Bason 1985; Coats et al. 1995). Coarse, sandy sediments from dredge spoils fail to retain moisture, nutrients, and organic matter, and may result in sparse, low biomass, marsh vegetation (Langis et al. 1991; Gibson et al. 1994). Without vegetative root support, sediments have a lower cohesive strength and erode easily, forming gradually sloping banks (Garofalo 1980; Coats et al. 1989, 1995; Haltiner et al. 1997). Hydrology also affects sediment deposition, erosion potential, and creek shape (Everts 1980; Coats et al. 1989). In turn, bank slope, elevation, and water depth can influence wetland plant growth, plant coverage, and other related habitat functions (Kentula et al. 1992). We recommend that marsh designs incorporate organic sediments that facilitate plant growth and maintain natural bank morphology, aspects of channel sinuosity that allow development of a variety of bank slopes (gradual depositional and steep erosional), and planning with regard to the local hydrologic landscape. We recommend that marsh designs seek to include a diversity of tidal creek morphologies and densities that imitate the hydrologic and structural heterogeneity of natural systems. In our study, networks of small intertidal (low-order) tributaries were absent in the design of constructed wetlands in Sweetwater Marsh. A review of several created wetland habitats in the southern California coast similarly show that most lack a natural diversity of landscape features, and generally have little intertidal area, minimal connectivity to natural salt marsh habitats, and low edge ratios (Zedler et al. 1997). Creation of tidal creeks with shallow shoreline slopes increases the amount of edge, and should improve the value of a created salt marsh by increasing the area of vegetated refuge with potential for important fish nursery and feeding functions (Minello et al. 1994). Other studies have suggested the lower habitat complexity o f constructed habitats lacking small creeks and rivulets may be reflected in the reduced density of some species (Havens et al. 1995), especially juveniles (Desmond et al. In press). Shallow creeks promote tidal access by fishes to low-elevation intertidal areas (Rozas et al. 1988), where primary production is high (Zedler 1980) and may contribute significantly to the higher trophic levels of the estuarine food web (Kwak and Zedler 1997). ASSESSMENT

Because hydrologic conditions are an important determinant to wetland structure and function

713

(Mitsch and Gosselink 1993), we recommend assessments of constructed channel performance be matched with appropriate reference sites by the hydrogeomorphic approach (Brinson and Rheinhardt 1996). Although the inclusion of multiple reference sites increased our ability to measure local variability in fish populations, the wide spectrum of channel sizes and hydrologic influences may have overshadowed differences in the fish biota attributable to habitat construction alone. Assessment is as important as plan design in habitat restoration projects because reference site choice may influence decisions regarding satisfaction of mitigation standards (Kentula et al. 1992; Brinson and Rheinhardt 1996; Zedler 1996b). Studies seeking to compare natural and restored sites should strike a balance between hydrogeomorphic subclassification of study sites (e.g., Brinson 1993), to reduce the complicating effects of habitat variability on species associations, and broader classifications that result in lost resolution due to more widely varying reference standards (Streever and Portier 1994; Brinson and Rheinhardt 1996). Constructed wetlands should be evaluated over many years against long-term data sets from multiple, local reference sites, rather than static restoration targets. During our study, we observed considerable interannual and inter-site variability in fish density estimates, with exceptionally high numbers colonizing one site (C4) in the first year after construction. Southern California's climate is characterized by environmental extremes, including episodic flooding and sedimentation events (Allen 1982; Onuf and Quammen 1983), which occurred within our sampling period (1993 and 1995). Run-off events that reduce wetland salinities and increase sedimentation rates have been correlated to both seasonal and interannual fluctuations in fish abundance and distribution (Onuf and Quammen 1983; Nordby and Zedler 1991; Yoklavich et al. 1991). The schooling nature offishes and their distinct seasonality may also contribute to highly variable density estimates. Finally, the biological significance of extremely high fish densities in newly constructed channels remains to be explained. Other monitoring projects have reported similar pulses, which may be attributed to altered tidal circulation patterns that enable algal growth, increased cover and food, and novel habitat formation (Williams et al. 1998). While we support the j u d g m e n t that the Sweetwater Marsh project complied with mitigation criteria, future mitigation assessments should be improved by considering factors beyond richness and density, both of which have shortcomings. Species richness depends on sample size, varies regionally, and conveys limited information on the relative

714


c o m p o s i t i o n o f i n d i v i d u a l species a n d t h e i r f u n c t i o n a l r o l e s (e.g., t r o p h i c level) w i t h i n n a t u r a l syst e m s ( F a u s c h et al. 1990). T o t a l d e n s i t y m a y p r o vide a p o o r m e a s u r e o f h a b i t a t f u n c t i o n b e c a u s e d a t a c a n b e a f f e c t e d by a n o v e r w h e l m i n g a b u n d a n c e o f a s c h o o l i n g species o r o p p o r t u n i s t i c species t o l e r a n t o f d e g r a d e d c o n d i t i o n s ( M i n e l l o a n d W e b b 1997). M o n i t o r i n g p r o j e c t s t h a t rely solely o n t h e s e s u m m a r y a s s e s s m e n t m e a s u r e s m a y provide m i s l e a d i n g i n f o r m a t i o n o n t h e relative success of creating functionally equivalent habitats within a g i v e n t i m e s p a n . T h e y also m a y m a s k real differe n c e s i n i n d i v i d u a l species h e a l t h , a s s e m b l a g e s t r u c t u r e , a n d c o m m u n i t y e c o l o g y ( F a u s c h et al. 1990; M i n e l l o a n d W e b b 1997). We r e c o m m e n d i n c l u s i o n o f i n f o r m a t i o n o n t h e p r e s e n c e o r relative a b u n d a n c e o f i n d i v i d u a l species, w h i c h have n a r r o w e r h a b i t a t r e q u i r e m e n t s ( K a r r 1981) a n d m a y p r o v i d e m o r e sensitive e v i d e n c e o f a c o n structed habitat's functional shortcomings. Summ a r y m e a s u r e s o f fish use (i.e., total density, diversity) s h o u l d b e s u p p l e m e n t e d with e s t i m a t e s o f c o m m u n i t y f u n c t i o n t h a t c o m p a r e f e e d i n g success (Moy a n d L e v i n 1991; S h r e f f l e r et al. t 9 9 2 ) , resid e n c e t i m e ( S h r e f f l e r et al. 1990), r e p r o d u c t i o n ( T r e x l e r 1995), a n d g r o w t h (Miller a n d S i m e n s t a d 1997) b e t w e e n n a t u r a l a n d c o n s t r u c t e d o r r e s t o r e d habitats. Large-scale, c o n t r o l l e d , e x p e r i m e n t a l m a n i p u l a t i o n s n e e d to b e i n t e g r a t e d i n t o r e s t o r a t i o n proj e c t s i n o r d e r to d e v e l o p b e t t e r w e t l a n d r e s t o r a t i o n m e t h o d s . H a b i t a t m i t i g a t i o n r e q u i r e m e n t s offer u n i q u e o p p o r t u n i t i e s for r e s e a r c h i n t o r e s t o r a t i o n e c o l o g y a n d o t h e r e c o l o g i c a l d i s c i p l i n e s (see A l l e n et al. 1997 a n d a s s o c i a t e d p a p e r s ) . F u t u r e s t u d i e s might explore appropriate restoration endpoints from a c o m m u n i t y ecology perspective, focusing o n t h e r e l a t i o n s h i p b e t w e e n w e t l a n d h a b i t a t struct u r e a n d c o m m u n i t y f u n c t i o n ( P a l m e r et al. 1997). R e g a r d i n g q u e s t i o n s o f l a n d s c a p e scale a n d patt e r n , we n e e d to i n v e s t i g a t e t h e i m p o r t a n c e o f h a b itat l i n k a g e s to r e s t o r e d sites a n d t h e i n f l u e n c e o f p r o x i m i t y to n a t u r a l m a r s h e s a n d a d j a c e n t ecosyst e m s ( S i m e n s t a d a n d T h o r n 1996; Bell et al. 1997; E h r e n f e l d a n d T o t h 1997). Finally, i n v e s t i g a t i o n s should e x a m i n e the role of habitat disturbance in t h e e s t a b l i s h m e n t o f n o n i n d i g e n o u s species a n d t h e i r i m p a c t o n n a t i v e species ( M e n g et al. 1994; Moyle a n d L i g h t 1996). ACKNOWI.EDGMENTS Many thanes to the following personnel for help in the field and laboratory: Mark Tucker, Julie Desmond, Jeff Kepper, Doug Gibson, Kathy Boyer,Jennifer Lewis, Dan Daft, Michelle Cordrey, and Michael Voss. We also wish to acknowledge the efforts of PERL employees involved in the collection of this monitoring data during previous years, including Tom Kwak, Bruce Nyden, John Boland, Chris Nordby, and 'Fed Griswold. Greg Noe and

Doug Deutschman assisted with multivariate data analysis and interpretation. Discussions with John Callaway and Julie Desmond, and comments by two anonymous reviewers substantially improved the text. We acknowledge funding from the California Department of Transportation and Pam Beare's facilitation of our work; we thank the United States Fish and Wildlife Service Refuge Managers Tom Alexander and Dean Rundle for providing access to the field site. LITERATURF. CITED ALLEN, E. B., W. W. COVINGTON,AND D. A. FALK. 1997. Developing the conceptual basis for restoration ecology. Restoration Ecology 5:275-276. AI.t.~:N, L. G. 1982. Seasonal abundance, composition, and productivity of the littoral fish assemblage in upper Newport Bay, California. Fishery Bulletin, United State., 80:769-789. AI ivy, L. G. 1985. A habitat analysis of the nearshore marine fishes from southern California. Bulletin of the Southern California Academy of Sciences 84:133--155. BAt:rz, D. M., C. RAKOC1NSKI,ANDJ. w. FIY.EGER.1993. Microhabitat u ~ by marsh-edge fishes in a Louisiana estuary. Fnvironmental Biology of Fishes 36:109-126. BELL, S. S., M. S. FONSECZA,AND L. B. MO'rl'EN. 1997. Linking restoration and landscape ecology. Restorat/onEco/ogy5:318--323. BOF~;H, D. E A~'~DR. E. TUR~'~ER.1984. Dependence of fishery species on salt marshes: The role of tbod and refuge. Estuaries 7:460-468. BORTONE, S. A. ANDJ. j. KIMY,EL. 1991. Environmental assessment and monitoring of artificial habitats, p. 177-234. In W. Seaman and L. M. Sprague (eds.), Artificial Habitats for Marine and Freshwater Fisheries. Academic Press, Inc., San Diego, California. BRINSON,M. M. ANDR. RHEINIIARDT.1996. The role of reference wetlands in functional assessment and mitigation. Ecological Applications 6:69-76. BROTHb:RS,E. B. 1975. The comparative ecology and behavior of three sympatric California gobies. Ph.D. Dissertation, University of California, San Diego, California. CAP,VEI~\', L. H. 1961. Salinity tolerances of some fishes of a southern California coastal lagoon. Copeia 1:32-39. CHAMBERS,J. R. 1992. Coastal degradation and fish population losses, p. 45-51. In R. H. Stroud (ed.), Stemming the Tide of Coastal Fish Habitat Loss. National Coalition for Marine Conservation, Inc., Savannah, Georgia. CLARK,B. M., B. A. BENN~;'rT,ANDS.J. LAMRER'rH.1996. Factors affecting spatial variability in seine net catches of fish in the surf zone of False Bay. South Africa. Marine Ecology Progress Ser/es 131:17-34. COATS,R., M. SWANSON,ANDP. WILLIAMS.1989. Hydrologic analysis for coastal wetland restoration. Environmental Management 13:715-727. DESMOND,J. s., G. D. WILLIAMS,ANDJ. B. ZEDLFR.In press. Fish u ~ of tidal creek habitats in two southern California saltmarshes. EcologicalEngineering. Enm;NrELD,J. G. ANDL. A. TO'rH. 1997. Restoration ecology and the ecosystem perspective. Restoration Ecology 5:307-317. EMMETF,R. L., S. L. STONE, S. A. HINTON,AND M. E. MONACO. 1991. Distribution and Abundance of Fishes and hwertebrates in West Coast Estuaries, Volume If. Species Life History Summaries. Estuarine Living Marine Resources Program Report No. 8. National Oceanic and Atmospheric Administration, National Ocean Survey, Strategic Environmental Assessments Division, Rockville, Maryland. EVERTS, C. H. 1980. A Method to Predict the Stable Geometry of a Channel Connecting an Enclosed I Iarbor and Navigable Waters. United States Army Corps of Engineers Coastal Engineering Research Center Technical Paper No. 80-6. Ft. Belvoir, Virginia. FAUSCH,K. D.,J. LYONS,J. R. KARR,A,'~DP. L. AN'GERMEIER.1990.


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Receivedfor consideratiom September 19, 1997 Acceptedfor publication, November 16, 1998